Project index

This is a bit of a discussion piece. It uses a hack where the index.html of generated doc dirs get a comment containing some data about a project. When you copy/symlink the documentation under what you want to be the master documentation directory and run `ex_doc --generate-project-index -o master_doc_dir` it scans for index.html files that contain the necessary documentation block and if found links to those files.

Example of how it looks (using some slight hackery on various projects to add description / split the main elixir repo into multiple docs):

![project_index](https://f.cloud.github.com/assets/4168875/1114220/7cfef67a-1a14-11e3-9ac0-09eedef6b300.png)

Not sure if this is the approach we want to take as it's a bit (or rather "rather") ugly. But it works and it would be fairly simple to add an "install-docs" task to Mix that would put/link docs under a standard directory and run "--generate-project-index" to update the docs.

Because this is a discussion piece I didn't bother adding tests just yet. If it looks like it's going to go in I'll add a commit with some tests.